Expansion vessel needs repressurising?

[BobRRR]

Hi all,

We've got a Worcester-Bosch 30CDi combi, it's approx 12 years old. Recently had a plumber out as our boiler was leaking from the RHS, turned out to be a split o-ring in the turbine sensor assembly.

A few days ago we've noticed a very very small leak from the LHS, which can only be a few drips a day, but there is evidence of white/green crud on one of the metal fittings inside the bottom of the boiler. Today, noticed that the pressure gauge was at 3.5 bar, and it's obviously been venting outside - now curiosity got the better of me, and looking right up inside shows a wet patch on the bottom of a black plastic piece where a copper pipe joins.

Took about 2 - 3 litres out of the system via a rad bleed valve, and pressure dropped to 1.5 bar, but the moment the heating started up again and had run for 20-30 mins it was back at 3 bar, and dripping outside.

I presume it is a gas safe engineer/plumber only job to attempt re-pressurising the expansion vessel?

 

[oldbutnotdead]

Before you worry about the ev, check that the filling loop isn't passing (if it's the trad flexi between 2 valves, disconnect one end of it).
If it took 3 litres to drop 2 bar of pressure I'd be surprised if the ev was deflated (unless you have a load of air trapped elsewhere in the system).

 

[BobRRR]

Before you worry about the ev, check that the filling loop isn't passing (if it's the trad flexi between 2 valves, disconnect one end of it).
If it took 3 litres to drop 2 bar of pressure I'd be surprised if the ev was deflated (unless you have a load of air trapped elsewhere in the system).

It's got the integral filling loop and the plastic key, of which is on the shelf so definitely don't think it's filling loop related (if you open the filling knob without the key in it just ****es out the bottom... ask me how I know).

Approx 2-3 litres, I was using a 500ml cup and had about 4-5 out of a rad, but probably closer to 2 as they weren't full.

 

[oldbutnotdead]

Ok. Get the system down to sensible pressure then turn off your cold water supply and fire it up (leave the cold tap open at the kitchen sink in case your main stoptap is also passing). Valves have moving parts and thus can fail. Repressurising an ev isn't per se a Gas Safe job BUT if getting to the valve on the ev involves removing the room seal cover of the boiler then it does become Gas Safe territory.

 

[BobRRR]

Hi oldbutnotdead,

I see what you mean with your steps, the mains water supply has been isolated on the bottom of the boiler and hot taps opened to relieve the pressure there.

If the pressure does not rise after the CH has been on and cycled, is that definitely a big repair, likely heat exchanger kind of deal (i've been doing some reading elsewhere)? If so, it might be beneficial for us to look at the Worcester Bosch care plan as the boiler is 6 months before it's 12 year birthday.

I took another 2L off it as it had spiked to 3.5 bar again (in about 2 hours) after the heating had been on, bringing it back down to about 1.5 bar. Will see in the morning once the heating has cycled back on again, and if no change, i'll open the mains water valve back up again and see if it pressurises.

Dogs danglies I didn't expect this!

 

[oldbutnotdead]

Hmm. Yes of course it's going to be difficult to separate the cold to dhw from the cold to top up heating pressure...see what results you get from the experiment first.
While you're Googling, find out if access to the ev in your particular boiler involves removal of the room seal, if it doesn't then that's your next task (and if you can DIY make sure you follow all the steps required to repressurise the thing). If it does then consider a WB fixed price repair- think it's £400 ish but does protect you against parts slingers experimenting at your expense

 

[BobRRR]

Hmm. Yes of course it's going to be difficult to separate the cold to dhw from the cold to top up heating pressure...see what results you get from the experiment first.
While you're Googling, find out if access to the ev in your particular boiler involves removal of the room seal, if it doesn't then that's your next task (and if you can DIY make sure you follow all the steps required to repressurise the thing). If it does then consider a WB fixed price repair- think it's £400 ish but does protect you against parts slingers experimenting at your expense

It's a Worcester 30CDi Combi, so I believe it is totally room sealed... i've been around when the cover has been off before, in the presence of a gas safe engineer, and I know the expansion vessel is the big red pillow shaped thing on the lhs.

Fingers crossed, i've also found BG do a fixed price repair with signup to 12 months repair contract, £5 cheaper pm than Worcester and no maximum initial repair cost from what I can see.

Edit: Quick test, i've left the hot water tap on (closest to the boiler) and so far the water that has dripped out of it is a bit... questionable, but we don't use this tap much! Have rigged up a clear plastic cup to capture what (if any), comes out when the heating comes on in the morning. Thought process is that if there is a mixing of CH and hot water, it will also go the other way, causing CH water to leak into the hot water circuit (but maybe i'm way off...?).

Edit 2: The filling loop on this boiler has a plastic key which has two o-rings and just completes the passageway (without the key in, there is an open hole in the filling circuit, so if it was leaking here it'd just run out of the bottom of the boiler as far as I know).

 

[Mottie]

Fingers crossed, i've also found BG do a fixed price repair with signup to 12 months repair contract, £5 cheaper pm than Worcester and no maximum initial repair cost from what I can see.

Be careful with that - don't know if they've changed things but I think it has to be checked first before they will take it on.

 

[BobRRR]

Pressure dropped to 0.8-0.9 over night. With the heating (and mains water back on for the past 6 hours), it's back at 1.5 now.

Aiui from reading the manuals, the mains water supply enters the smaller (less expensive) plate heat exchanger at the back of the boiler, where it is heated by the CH water run through the diverter valve from the main heat exchanger.

We're going to keep an eye on it over the next couple of days whilst we source a plumber to come and have a look. Boiler is due a service anyway!

Edit: I never posted this earlier, but as of 16:30hrs we're back at 2-2.5 bar (the heating has come back on again this afternoon), but it's not going any higher so far. The mains water is off again to the boiler just to see if the pressure skyrockets again without it on.

 

[BobRRR]

After being quoted north of £300 for a fix from local places (all suspecting the plate heat exchanger), we decided to try British Gas's offer of signing up to a year of home serve and having it fixed for free (so £300 for the year).

A plate heat exchanger, a plastic manifold and PRV all changed, expansion vessel pumped up as was empty (but not leaking), a very professional attitude from the engineer, and boiler is now working spot on!

Might be worth a shot if someone has a similar problem, as previously said, it prevents people firing the parts cannon at your expense.

 

[DP]

Might be worth a shot if someone has a similar problem, as previously said, it prevents people firing the parts cannon at your expense.

What makes you think fixing boilers by changing parts is the method used by every heating engineer?

I assume the person who carried out the repair cleared the condensate trap, checked manifold pressure and carried out FGA is as per the book rather than 0.004 ratio as often done.

If that boiler was serviced, properly as the manual requires you would not be posting here asking for guidance as to what to do. Pressure loss on expansion vessel is a common occurrence of slippage of service schedule unless the vessel is goosed

Here is a heads up for you. Look at the service pages of the manual. Then see if all the requirements are carried out when the boiler is serviced

 

[BobRRR]

What makes you think fixing boilers by changing parts is the method used by every heating engineer?

I assume the person who carried out the repair cleared the condensate trap, checked manifold pressure and carried out FGA is as per the book rather than 0.004 ratio as often done.

If that boiler was serviced, properly as the manual requires you would not be posting here asking for guidance as to what to do. Pressure loss on expansion vessel is a common occurrence of slippage of service schedule unless the vessel is goosed

Here is a heads up for you. Look at the service pages of the manual. Then see if all the requirements are carried out when the boiler is serviced

Thanks for your kind words. The parts cannon comment was directed at the previous poster. I'm not competent so cannot do any servicing or other maintenance myself as the whole boiler is room sealed.

Servicing was interrupted by covid as people in the household are classed as vulnerable and absolutely did not want outsiders in the house. Understandably so.

It will be serviced every year now as it should be.